Читать книгу Molecular Mechanisms of Photosynthesis - Robert E. Blankenship - Страница 15

Photosynthesis is carried out by a wide variety of organisms. In all cases, lipid bilayer membranes are critical to the early stages of energy storage, such that photosynthesis must be viewed as a process that is at heart membrane‐based. The early processes of photosynthesis are carried out by pigment‐containing proteins that are integrally associated with the membrane. Later stages of the process that occur on a slower (e.g. millisecond) time scale are mediated by proteins that are freely diffusible in the aqueous phase.

In eukaryotic photosynthetic cells, photosynthesis is localized in subcellular structures known as chloroplasts (Fig. 1.2). The chloroplast contains all the chlorophyll pigments and, in most organisms, carries out all the main phases of the process of photosynthesis. Synthesis of sucrose and some other carbon metabolism reactions require extrachloroplastic enzymes. Chloroplasts are about the size of bacteria, a few micrometers in diameter. In fact, chloroplasts were derived long ago from symbiotic bacteria that became integrated into the cell and eventually lost their independence, a process known as endosymbiosis (see Chapter 12). Even today, they retain traces of their bacterial heritage, including their own DNA, although much of the genetic information needed to build the photosynthetic apparatus now resides in DNA located in the nucleus.

Figure 1.2 Exploding diagram of the photosynthetic apparatus of a typical higher plant. The first expansion bubble shows a cross‐section of a leaf, with the different types of cells; the dark spots are the chloroplasts. The second bubble is a chloroplast; the thylakoid membranes are the dark lines; the stroma is the stippled area. The third bubble shows a grana stack of thylakoids. The fourth bubble shows a schematic picture of the molecular structure of the thylakoid membrane, with a reaction center flanked by antenna complexes.

Source: Courtesy of Aileen Taguchi.

An extensive membrane system is found within the chloroplast, and all the chlorophylls and other pigments are found associated with these membranes, which are known as thylakoids, or sometimes called lamellae. In typical higher plant chloroplasts, most of the thylakoids are closely associated in stacks and are known as grana thylakoid membranes, while those that are not stacked are known as stroma thylakoid membranes. The thylakoid membranes are the sites of light absorption and the early or primary reactions that first transform light energy into chemical energy. The nonmembranous aqueous interior of the chloroplast is known as the stroma. The stroma contains soluble enzymes and is the site of the carbon metabolism reactions that ultimately give rise to products that can be exported from the chloroplast and used elsewhere in the plant to support other cellular processes.

In prokaryotic photosynthetic organisms, the early steps of photosynthesis take place on specialized membranes that are derived from the cell's cytoplasmic membrane. In these organisms, the carbon metabolism reactions take place in the cell cytoplasm, along with all the other reactions that make up the cell's metabolism.